Various functions of forming a machined groove (a grooving process), increasing the width of the machined groove (a shoulder grooving process) and the like are required for cutting inserts, particularly cutting inserts for the grooving process. Therefore, various attempts have been made to provide a cutting insert configured to have shapes suitable for the individual functions. Japanese Unexamined Patent Publication No. 9-174308 discloses a cutting insert made up of a shaft part and a cutting head. The cutting head includes a major cutting edge having an edge part held down into a curve-like shape, and a pair of ridges extending in a longitudinal direction on a top surface of the cutting head. According to this configuration, chips generated by the major cutting edge in the grooving process are discharged by being curved between the pair of ridges and compressed in a width direction of the chips.
However, this type of cutting insert needs to ensure a large distance between the pair of ridges in order to satisfactorily curve the chips in the width direction in the grooving process as described above. On the other hand, only an end side of the major cutting edge is used to increase the width of a machined groove in the shoulder grooving process. It is therefore necessary to ensure a longer length of the cutting edge located further outward than the ridge part. The foregoing cutting insert is configured so that the distance between the pair of ridges is relatively large and the width of the top surface located further outward than the ridges is small. Consequently, it may be difficult to obtain sufficient performance in the shoulder grooving process.
Also in the foregoing cutting insert, the ridges are formed to become higher as separating from the major cutting edge. Therefore, the chips generated by the major cutting edge can be guided by the ridges, however, it may be difficult to control the chips because the curling diameter of the chips is increased. Hence, the discharge direction of the chips becomes unstable, that is, the chips travel while causing lateral vibration. As a result, the curled chips are entwined with each other, and the chips are contacted with a machined surface, thereby deteriorating chip discharge performance and damaging a machined wall surface.